1. science
2. physics

How the Sun Shines∗
What makes the sun shine? How does the sun produce the vast amount of energy
necessary to support life on earth? These questions challenged scientists for a hundred and
fifty years, beginning in the middle of the nineteenth century. Theoretical physicists battled
geologists and evolutionary biologists in a heated controversy over who had the correct
answer.
Why was there so much fuss about this scientific puzzle? The nineteenth-century astronomer John Herschel described eloquently the fundamental role of sunshine in all of
human life in his 1833 Treatise on Astronomy:
The sun’s rays are the ultimate source of almost every motion which takes place
on the surface of the earth. By its heat are produced all winds,...By their vivifying
action vegetables are elaborated from inorganic matter, and become, in their
turn, the support of animals and of man, and the sources of those great deposits
of dynamical efficiency which are laid up for human use in our coal strata.
FIG. 1. Sunshine makes life possible on earth.
In this essay, we shall review from an historical perspective the development of our
understanding of how the sun (the nearest star) shines, beginning in the following section
∗ Published
in the Nobel e-Museum, http://www.nobel.se/, June, 2000, and in astro-ph/0009259.
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with the nineteenth-century controversy over the age of the sun. In later sections, we shall
see how seemingly unrelated discoveries in fundamental physics led to a theory of nuclear
energy generation in stars that resolved the controversy over the age of the sun and explained
the origin of solar radiation. In the section just before the summary, we shall discuss how
experiments that were designed to test the theory of nuclear energy generation in stars
revealed a new mystery, the Mystery of the Missing Neutrinos.
I. THE AGE OF THE SUN
How old is the sun? How does the sun shine? These questions are two sides of the same
coin, as we shall see.
The rate at which the sun is radiating energy is easily computed by using the measured
rate at which energy reaches the earth’s surface and the distance between the earth and
the sun. The total energy that the sun has radiated away over its lifetime is approximately
the product of the current rate at which energy is being emitted, which is called the solar
luminosity, times the age of the sun.
The older the sun is, the greater the total amount of radiated solar energy. The greater
the radiated energy, or the larger the age of the sun, the more difficult it is to find an
explanation of the source of solar energy.
To better appreciate how difficult it is to find an explanation, let us consider a specific
illustration of the enormous rate at which the sun radiates energy. Suppose we put a cubic
centimeter of ice outside on a summer day in such a way that all of the sunshine is absorbed
by the ice. Even at the great distance between the earth and the sun, sunshine will melt
the ice cube in about 40 minutes. Since this would happen anywhere in space at the earth’s
distance from the sun, a huge spherical shell of ice centered on the sun and 300 million km
(200 million miles) in diameter would be melted in the same time. Or, shrinking the same
amount of ice down to the surface of the sun, we can calculate that an area ten thousand
times the area of the earth’s surface and about half a kilometer (0.3 mile) thick would also
be melted in 40 minutes by the energy pouring out of the sun.
In this section, we shall discuss how nineteenth-century scientists tried to determine the
source of solar energy, using the solar age as a clue.
A. Conflicting Estimates of the Solar Age
The energy source for solar radiation was believed by nineteenth-century physicists to be
gravitation. In an influential lecture in 1854, Hermann von Helmholtz, a German professor of
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physiology who became a distinguished researcher and physics professor, proposed that the
origin of the sun’s enormous radiated energy is the gravitational contraction of a large mass.
Somewhat earlier, in the 1840’s, J. R. Mayer (another German physician) and J. J. Waterson
had also suggested that the origin of solar radiation is the conversion of gravitational energy
into heat1 .
Biologists and geologists considered the effects of solar radiation, while physicists concentrated on the origin of the radiated energy. In 1859, Charles Darwin, in the first edition
of On The Origin of the Species by Natural Selection, made a crude calculation of the age of
the earth by estimating how long it would take erosion occurring at the current observed rate
to wash away the Weald, a great valley that stretches between the North and South Downs
across the south of England. He obtained a number for the “denudation of the Weald” in the
range of 300 million years, apparently long enough for natural selection to have produced
the astounding range of species that exist on earth.
As Herschel stressed, the sun’s heat is responsible for life and for most geological evolution
on earth. Hence, Darwin’s estimate of a minimum age for geological activity on the earth
implied a minimum estimate for the amount of energy that the sun has radiated.
Firmly opposed to Darwinian natural selection, William Thompson, later Lord Kelvin,
was a professor at the University of Glasgow and one of the great physicists of the nineteenth century. In addition to his many contributions to applied science and to engineering,
Thompson formulated the second law of thermodynamics and set up the absolute temperature scale, which was subsequently named the Kelvin scale in his honor. The second law
of thermodynamics states that heat naturally flows from a hotter to a colder body, not the
opposite. Thompson therefore realized that the sun and the earth must get colder unless
there is an external energy source and that eventually the earth will become too cold to
support life.
Kelvin, like Helmholtz, was convinced that the sun’s luminosity was produced by the
conversion of gravitational energy into heat. In an early (1854) version of this idea, Kelvin
suggested that the sun’s heat might be produced continually by the impact of meteors falling
1 von
Helmholtz and Mayer were two of the codiscoverers of the law of conservation of
energy. This law states that energy can be transformed from one form to another but the
total amount is always conserved. Conservation of energy is a basic principle of modern
physics that is used in analyzing the very smallest (sub-atomic) domains and the largest
known structure (the universe), and just about everything in between. We shall see later
that Einstein’s generalization of the law of conservation of energy was a key ingredient in
understanding the origin of solar radiation. The application of conservation of energy to
radioactivity revealed the existence of neutrinos.
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onto its surface. Kelvin was forced by astronomical evidence to modify his hypothesis and he
then argued that the primary source of the energy available to the sun was the gravitational
energy of the primordial meteors from which it was formed.
Thus, with great authority and eloquence Lord Kelvin declared in 1862:
That some form of the meteoric theory is certainly the true and complete explanation of solar heat can scarcely be doubted, when the following reasons are
considered: (1) No other natural explanation, except by chemical action, can
be conceived. (2) The chemical theory is quite insufficient, because the most
energetic chemical action we know, taking place between substances amounting
to the whole sun’s mass, would only generate about 3,000 years’ heat. (3) There
is no difficulty in accounting for 20,000,000 years’ heat by the meteoric theory.
Kelvin continued by attacking Darwin’s estimate directly, asking rhetorically:
What then are we to think of such geological estimates as [Darwin’s] 300, 000, 000
years for the “denudation of the Weald”?
Believing Darwin was wrong in his estimate of the age of the earth, Kelvin also believed
that Darwin was wrong about the time available for natural selection to operate.
Lord Kelvin estimated the lifetime of the sun, and by implication the earth, as follows.
He calculated the gravitational energy of an object with a mass equal to the sun’s mass and
a radius equal to the sun’s radius and divided the result by the rate at which the sun radiates
away energy. This calculation yielded a lifetime of only 30 million years. The corresponding
estimate for the lifetime sustainable by chemical energy was much smaller because chemical
processes release very little energy.
B. Who was right?
As we have just seen, in the nineteenth century you could get very different estimates
for the age of the sun, depending upon whom you asked. Prominent theoretical physicists
argued, based upon the sources of energy that were known at that time, that the sun was at
most a few tens of million years old. Many geologists and biologists concluded that the sun
must have been shining for at least several hundreds of millions of years in order to account
for geological changes and the evolution of living things, both of which depend critically
upon energy from the sun. Thus the age of the sun, and the origin of solar energy, were
important questions not only for physics and astronomy, but also for geology and biology.
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Darwin was so shaken by the power of Kelvin’s analysis and by the authority of his
theoretical expertise that in the last editions of On The Origin of the Species he eliminated all
mention of specific time scales. He wrote in 1869 to Alfred Russel Wallace, the codiscoverer
of natural selection, complaining about Lord Kelvin:
Thompson’s views on the recent age of the world have been for some time one
of my sorest troubles.
Today we know that Lord Kelvin was wrong and the geologists and evolutionary biologists
were right. Radioactive dating of meteorites shows that the sun is 4.6 billion years old.
What was wrong with Kelvin’s analysis? An analogy may help. Suppose a friend observed you using your computer and tried to figure out how long the computer had been
operating. A plausible estimate might be no more than a few hours, since that is the maximum length of time over which a battery could supply the required amount of power. The
flaw in this analysis is the assumption that your computer is necessarily powered by a battery. The estimate of a few hours could be wrong if you computer were operated from an
electrical power outlet in the wall. The assumption that a battery supplies the power for
your computer is analogous to Lord Kelvin’s assumption that gravitational energy powers
the sun.
Since nineteenth century theoretical physicists did not know about the possibility of
transforming nuclear mass into energy, they calculated a maximum age for the sun that
was too short. Nevertheless, Kelvin and his colleagues made a lasting contribution to the
sciences of astronomy, geology, and biology by insisting on the principle that valid inferences
in all fields of research must be consistent with the fundamental laws of physics.
We will now discuss some of the landmark developments in the understanding of how
nuclear mass is used as the fuel for stars.
II. A GLIMPSE OF THE SOLUTION
The turning point in the battle between theoretical physicists and empirical geologists
and biologists occurred in 1896. In the course of an experiment designed to study x-rays
discovered the previous year by Wilhelm Roentgen, Henri Becquerel stored some uraniumcovered plates in a desk drawer next to photographic plates wrapped in dark paper. Because it was cloudy in Paris for a couple of days, Becquerel was not able to “energize” his
photographic plates by exposing them to sunlight as he had intended. On developing the
photographic plates, he found to his surprise strong images of his uranium crystals. He had
discovered natural radioactivity, due to nuclear transformations of uranium.
5
The significance of Becquerel’s discovery became apparent in 1903, when Pierre Curie
and his young assistant, Albert Laborde, announced that radium salts constantly release
heat. The most extraordinary aspect of this new discovery was that radium radiated heat
without cooling down to the temperature of its surroundings. The radiation from radium
revealed a previously unknown source of energy. William Wilson and George Darwin almost
immediately proposed that radioactivity might be the source of the sun’s radiated energy.
The young prince of experimental physics, Ernest Rutherford, then a professor of physics
at McGill University in Montreal, discovered the enormous energy released by alpha particle
radiation from radioactive substances. In 1904, he announced:
The discovery of the radio-active elements, which in their disintegration liberate
enormous amounts of energy, thus increases the possible limit of the duration of
life on this planet, and allows the time claimed by the geologist and biologist for
the process of evolution.
The discovery of radioactivity opened up the possibility that nuclear energy might be the
origin of solar radiation. This development freed theorists from relying in their calculations
on gravitational energy. However, subsequent astronomical observations showed that the sun
does not contain a lot of radioactive materials, but instead is mostly hydrogen in gaseous
form. Moreover, the rate at which radioactivity delivers energy does not depend on the stellar
temperature, while observations of stars suggested that the amount of energy radiated by
a star does depend sensitively upon the star’s interior temperature. Something other than
radioactivity is required to release nuclear energy within a star.
In the next sections, we shall trace the steps that led to what we now believe is the
correct understanding of how stars shine.
III. THE DIRECTION ESTABLISHED
The next fundamental advance came once again from an unexpected direction. In 1905,
Albert Einstein derived his famous relation between mass and energy, E = mc2 , as a consequence of the special theory of relativity. Einstein’s equation showed that a tiny amount
of mass could, in principle, be converted into a tremendous amount of energy. His relation generalized and extended the nineteenth century law of conservation of energy of von
Helmholtz and Mayer to include the conversion of mass into energy.
What was the connection between Einstein’s equation and the energy source of the sun?
The answer was not obvious. Astronomers did their part by defining the constraints that
observations of stars imposed on possible explanations of stellar energy generation. In 1919,
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Henry Norris Russell, the leading theoretical astronomer in the United States, summarized
in a concise form the astronomical hints on the nature of the stellar energy source. Russell
stressed that the most important clue was the high temperature in the interiors of stars.
F. W. Aston discovered in 1920 the key experimental element in the puzzle. He made
precise measurements of the masses of many different atoms, among them hydrogen and
helium. Aston found that four hydrogen nuclei were heavier than a helium nucleus. This
was not the principal goal of the experiments he performed, which were motivated in large
part by looking for isotopes of neon.
FIG. 2. Aston showed in 1920 that four hydrogen nuclei are heavier than a helium nucleus.
The importance of Aston’s measurements was immediately recognized by Sir Arthur
Eddington, the brilliant English astrophysicist. Eddington argued in his 1920 presidential
address to the British Association for the Advancement of Science that Aston’s measurement
of the mass difference between hydrogen and helium meant that the sun could shine by converting hydrogen atoms to helium. This burning of hydrogen into helium would (according
to Einstein’s relation between mass and energy) release about 0.7% of the mass equivalent
of the energy. In principle, this could allow the sun to shine for about a 100 billion years.
In a frighteningly prescient insight, Eddington went on to remark about the connection
between stellar energy generation and the future of humanity:
If, indeed, the sub-atomic energy in the stars is being freely used to maintain
their great furnaces, it seems to bring a little nearer to fulfillment our dream
of controlling this latent power for the well-being of the human race—or for its
suicide.
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IV. UNDERSTANDING THE PROCESS
The next major step in understanding how stars produce energy from nuclear burning,
resulted from applying quantum mechanics to the explanation of nuclear radioactivity. This
application was made without any reference to what happens in stars. According to classical
physics, two particles with the same sign of electrical charge will repel each other, as if they
were repulsed by a mutual recognition of ‘bad breath’. Classically, the probability that two
positively charged particles get very close together is zero. But, some things that cannot
happen in classical physics can occur in the real world which is described on a microscopic
scale by quantum mechanics.
In 1928, George Gamow, the great Russian-American theoretical physicist, derived a
quantum-mechanical formula that gave a non-zero probability of two charged particles overcoming their mutual electrostatic repulsion and coming very close together. This quantum
mechanical probability is now universally known as the “Gamow factor.” It is widely used
to explain the measured rates of certain radioactive decays.
In the decade that followed Gamow’s epochal work, Atkinson and Houtermans and later
Gamow and Teller used the Gamow factor to derive the rate at which nuclear reactions would
proceed at the high temperatures believed to exist in the interiors of stars. The Gamow
factor was needed in order to estimate how often two nuclei with the same sign of electrical
charge would get close enough together to fuse and thereby generate energy according to
Einstein’s relation between excess mass and energy release.
In 1938, C. F. von Weizs¨acker came close to solving the problem of how some stars shine.
He discovered a nuclear cycle, now known as the carbon-nitrogen-oxygen (CNO) cycle (see
Appendix A), in which hydrogen nuclei could be burned using carbon as a catalyst. However,
von Weizs¨acker did not investigate the rate at which energy would be produced in a star by
the CNO cycle nor did he study the crucial dependence upon stellar temperature.
By April 1938, it was almost as if the scientific stage had been intentionally set for the
entry of Hans Bethe, the acknowledged master of nuclear physics. Professor Bethe had
just completed a classic set of three papers in which he reviewed and analyzed all that
was then known about nuclear physics. These works were known among his colleagues as
“Bethe’s bible.” Gamow assembled a small conference of physicists and astrophysicists in
Washington, D. C. to discuss the state of knowledge, and the unsolved problems, concerning
the internal constitution of the stars.
In the course of the next six months or so, Bethe worked out the basic nuclear processes
by which hydrogen is burned (fused) into helium in stellar interiors. Hydrogen is the most
abundant constituent of the sun and similar stars, and indeed the most abundant element
in the universe.
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Bethe described the results of his calculations in a paper entitled “Energy Production in
Stars,” which is awesome to read. He authoritatively analyzed the different possibilities for
reactions that burn nuclei and selected as most important the two processes that we now
believe are responsible for sunshine. One process, the so-called p − p chain (see Appendix
B), builds helium out of hydrogen and is the dominant energy source in stars like the sun
and less massive stars.
The CNO cycle, the second process which was also considered by von Weizs¨acker, is most
important in stars that are more massive than the sun. Bethe used his results to estimate
the central temperature of the sun and obtained a value that is within 20% of what we
currently believe is the correct value (16 million degrees Kelvin)2 . Moreover, he showed that
his calculations led to a relation between stellar mass and stellar luminosity that was in
satisfactory agreement with the available astronomical observations.
In the first two decades after the end of the second world war, many important details
were added to Bethe’s theory of nuclear burning in stars. Distinguished physicists and
astrophysicists, especially A. G. W. Cameron, W. A. Fowler, F. Hoyle, E. E. Salpeter, M.
Schwarzschild, and their experimental colleagues, returned eagerly to the question of how
stars like the sun generate energy. From Bethe’s work, the answer was known in principle:
the sun produces the energy it radiates by burning hydrogen. According to this theory, the
solar interior is a sort of controlled thermonuclear bomb on a giant scale3 . The theory leads
to the successful calculation of the observed luminosities of stars similar to the sun and
provides the basis for our current understanding of how stars shine and evolve over time.
The idea that nuclear fusion powers stars is one of the cornerstones of modern astronomy
and is used routinely by scientists in interpreting observations of stars and galaxies.
W. A. Fowler,Willy as he was universally known, led a team of colleagues in his Caltech
Kellogg Laboratory and inspired physicists throughout the world to measure or calculate
the most important details of the p − p chain and the CNO cycle. There was plenty of
work to do and the experiments and the calculations were difficult. But, the work got done
because understanding the specifics of solar energy generation was so interesting. Most of
2 According
to the modern theory of stellar evolution, the sun is heated to the enormous
temperatures at which nuclear fusion can occur by gravitational energy released as the solar
mass contracts from an initially large gas cloud. Thus Kelvin and other nineteenth-century
physicists were partially right; the release of gravitational energy ignited nuclear energy
generation in the sun.
3 The
sensitive dependence of the Gamow factor upon the relative energy of the two charged
particles is, we now understand, what provides the temperature “thermostat” for stars.
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the efforts of Fowler and his colleagues M. Burbidge, G. R. Burbidge, F. Hoyle, and A. G.
W. Cameron) soon shifted to the problem of how the heavy elements, which are needed for
life, are produced in stars.
V. TESTING THE HYPOTHESIS OF NUCLEAR BURNING
Science progresses as a result of the clash between theory and experiment, between
speculation and measurement. Eddington, in the same lecture in which he first discussed
the burning of hydrogen nuclei in stars, remarked:
I suppose that the applied mathematician whose theory has just passed one
still more stringent test by observation ought not to feel satisfaction, but rather
disappointment—“Foiled again! This time I had hoped to find a discordance
which would throw light on the points where my model could be improved.”
Is there any way to test the theory that the sun shines because very deep in its interior
hydrogen is burned into helium? At first thought, it would seem impossible to make a
direct test of the nuclear burning hypothesis. Light takes about forty thousand years to leak
out from the center of the sun to the surface and when it finally emerges in the outermost
regions, light mainly tells us about the conditions in those outer regions. Nevertheless, there
is a way of “seeing” into the solar interior with neutrinos, exotic particles discovered while
trying to understand a different mystery4 .
A. Discovery, Confirmation, and Surprise
A neutrino is a sub-atomic particle that interacts weakly with matter and travels at a
speed that is essentially the speed of light. Neutrinos are produced in stars when hydrogen
4 The
existence of neutrinos was first proposed by Wolfgang Pauli in a 1930 letter to his
physics colleagues as a ”desperate way out” of the apparent non-conservation of energy in
certain radioactive decays (called β-decays) in which electrons were emitted. According
to Pauli’s hypothesis, which he put forward very hesitantly, neutrinos are elusive particles
which escape with the missing energy in β-decays. The mathematical theory of β-decay was
formulated by Enrico Fermi in 1934 in a paper which was rejected by the journal Nature
because “it contained speculations too remote from reality to be of interest to the reader.”
Neutrinos from a nuclear reactor were first detected by Clyde Cowan and Fred Reines in
1956.
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FIG. 3. This figure is a cross section of the sun. The features that are usually studied by astronomers
with normal telescopes that detect light are labeled on the outside, e.g., sunspot and prominences. Neutrinos
enable us to look deep inside the sun, into the solar core where nuclear burning occurs.
nuclei are burned to helium nuclei; neutrinos are also produced on earth in particle accelerators, in nuclear reactors, and in natural radioactivity. Based upon the work of Hans Bethe
and his colleagues, we believe that the process by which stars like the sun generate energy
can be symbolized by the relation,
41 H −→ 4 He + 2e+ + 2νe + energy,
(1)
in which four hydrogen nuclei (1 H, protons) are burned into a single helium nucleus (4 He, α
particle) plus two positive electrons (e+ ) and two neutrinos (νe ) plus energy. This process
releases energy to the star since, as Aston showed, four hydrogen atoms are heavier than
one helium atom. The same set of nuclear reactions that supply the energy of the sun’s
radiation also produce neutrinos that can be searched for in the laboratory.
Because of their weak interactions, neutrinos are difficult to detect. How difficult? A
solar neutrino passing through the entire earth has less than one chance in a thousand
billion of being stopped by terrestrial matter. According to standard theory, about a hundred
billion solar neutrinos pass through your thumbnail every second and you don’t notice them.
Neutrinos can travel unaffected through iron as far as light can travel in a hundred years
through empty space.
In 1964, Raymond Davis Jr. and I proposed that an experiment with 100, 000 gallons
of cleaning fluid (perchloroethylene, which is mostly composed of chlorine) could provide a
critical test of the idea that nuclear fusion reactions are the ultimate source of solar radiation. We argued that, if our understanding of nuclear processes in the interior of the sun
was correct, then solar neutrinos would be captured at a rate Davis could measure with
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a large tank filled with cleaning fluid. When neutrinos interact with chlorine, they occasionally produce a radioactive isotope of argon. Davis had shown previously that he could
extract tiny amounts of neutrino-produced argon from large quantities of perchloroethylene.
To do the solar neutrino experiment, he had to be spectacularly clever since according to
my calculations, only a few atoms would be produced per week in a huge Olympic-sized
swimming pool of cleaning fluid.
Our sole motivation for urging this experiment was to use neutrinos to:
enable us to see into the interior of a star and thus verify directly the hypothesis
of nuclear energy generation in stars.
As we shall see, Davis and I did not anticipate some of the most interesting aspects of this
proposal.
Davis performed the experiment and in 1968 announced the first results. He measured
fewer neutrinos than I predicted. As the experiment and the theory were refined, the disagreement appeared more robust. Scientists rejoiced that solar neutrinos were detected but
worried why there were fewer neutrinos than predicted.
What was wrong? Was our understanding of how the sun shines incorrect? Had I made
an error in calculating the rate at which solar neutrinos would be captured in Davis’s tank?
Was the experiment wrong? Or, did something happen to the neutrinos after they were
created in the sun?
Over the next twenty years, many different possibilities were examined by hundreds, and
perhaps thousands, of physicists, chemists, and astronomers5 . Both the experiment and the
theoretical calculation appeared to be correct.
Once again experiment rescued pure thought.
In 1986, Japanese physicists led by
Masatoshi Koshiba and Yoji Totsuka, together with their American colleagues, Eugene Beier
and Alfred Mann, reinstrumented a huge tank of water designed to measure the stability of
matter. The experimentalists increased the sensitivity of their detector so that it could also
serve as a large underground observatory of solar neutrinos. Their goal was to explore the
reason for the quantitative disagreement between the predicted and the measured rates in
the chlorine experiment.
The new experiment (called Kamiokande) in the Japanese Alps also detected solar neutrinos. Moreover, the Kamiokande experiment confirmed that the neutrino rate was less
5 Perhaps
the most imaginative proposal was made by Stephen Hawking, who suggested
that the central region of the sun might contain a small black hole and that this could be
the reason why the number of neutrinos observed is less than the predicted number.
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than predicted by standard physics and standard models of the sun and demonstrated that
the detected neutrinos came from the sun. Subsequently, experiments in Russia (called
SAGE, led by V. Gavrin), in Italy (GALLEX and later GNO led by T. Kirsten and E.
Belotti, respectively), and again in Japan (Super-Kamiokande, led by Y. Totsuka and Y.
Suzuki), each with different characteristics, all observed neutrinos from the solar interior. In
each detector, the number of neutrinos observed was somewhat lower than standard theory
predicted.
What do all of these experimental results mean?
Neutrinos produced in the center of the sun have been detected in five experiments. Their
detection shows directly that the source of the energy that the sun radiates is the fusion
of hydrogen nuclei in the solar interior. The nineteenth century debate between theoretical
physicists, geologists, and biologists has been settled empirically.
From an astrophysical perspective, the agreement between neutrino observations and
theory is good. The observed energies of the solar neutrinos match the values predicted by
theory. The rates at which neutrinos are detected are less than predicted but not by a large
factor. The predicted neutrino arrival rate at the earth depends approximately upon the
25th power of the central temperature of the sun, T × T × ...T (25 factors of the temperature
T ). The agreement that has been achieved (agreement within a factor of three) shows that
we have empirically measured the central temperature of the sun to an accuracy of a few
percent. Incidentally, if someone had told me in 1964 that number of neutrinos observed
from the sun would be within a factor of three of the predicted value, I would have been
astonished and delighted.
In fact, the agreement between normal astronomical observations (using light rather than
neutrinos) and theoretical calculations of solar characteristics is much more precise. Studies
of the internal structure of the sun using the solar equivalent of terrestrial seismology (i.
e., observations of solar vibrations) show that the predictions of the standard solar model
for the temperatures in the central regions of the sun are consistent with the observations
to an accuracy of at least 0.1%. In this standard model, the current age of the sun is
five billion years, which is consistent with the minimum estimate of the sun’s age made by
nineteenth-century geologists and biologists (a few hundred million years).
Given that the theoretical models of the sun describe astronomical observations accurately, what can explain the disagreement by a factor of two or three between the measured
and the predicted solar neutrino rates?
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B. New physics
Physicists and astronomers were once again forced to reexamine their theories. This
time, the discrepancy was not between different estimates of the sun’s age, but rather between predictions based upon a widely accepted theory and direct measurements of particles
produced by nuclear burning in the sun’s interior. This situation was sometimes referred to
as the Mystery of the Missing Neutrinos or, in language that sounded more scientific, the
Solar Neutrino Problem.
As early as 1969, two scientists working in Russia, Bruno Pontecorvo and Vladimir
Gribov, proposed that the discrepancy between standard theory and the first solar neutrino
experiment could be due to an inadequacy in the textbook description of particle physics,
rather than in the standard solar model. (Incidentally, Pontecorvo was the first person to
propose using a chlorine detector to study neutrinos.) Gribov and Pontecorvo suggested
that neutrinos suffer from a multiple personality disorder, that they oscillate back and forth
between different states or types.
According to the suggestion of Gribov and Pontecorvo, neutrinos are produced in the
sun in a mixture of individual states, a sort of split personality. The individual states
have different, small masses, rather than the zero masses attributed to them by standard
particle theory. As they travel to the earth from the sun, neutrinos oscillate between the
easier-to-detect neutrino state and the more difficult-to-detect neutrino state. The chlorine
experiment only detects neutrinos in the easier-to-observe state. If many of the neutrinos
arrive at earth in the state that is difficult to observe, then they are not counted. It is as
if some or many of the neutrinos have vanished, which can explain the apparent mystery of
the missing neutrinos.
Building upon this idea, Lincoln Wolfenstein in 1978 and Stanislav Mikheyev and Alexei
Smirnov in 1985 showed that the effects of matter on neutrinos moving through the sun
might increase the oscillation probability of the neutrinos if Nature has chosen to give them
masses in a particular range.
Neutrinos are also produced by the collisions of cosmic ray particles with other particles in
the earth’s atmosphere. In 1998, the Super-Kamiokande team of experimentalists announced
that they had observed oscillations among atmospheric neutrinos. This finding provided
indirect support for the theoretical suggestion that solar neutrinos oscillate among different
states. Many scientists working in the field of solar neutrinos believe that, in retrospect, we
have had evidence for oscillations of solar neutrinos since 1968.
But, we do not yet know what causes the multiple personality disorder of solar neutrinos.
The answer to this question may provide a clue to physics beyond the current standard
models of sub-atomic particles. Does the change of identity occur while the neutrinos are
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traveling to the earth from the sun, as originally proposed by Gribov and Pontecorvo? Or
does matter cause solar neutrinos to “flip out”? Experiments are underway in Canada,
Italy (three experiments), Japan (two experiments), Russia, and the United States that are
attempting to determine the cause of the oscillations of solar neutrinos, by finding out how
much they weigh and how they transform from one type to another. Non-zero neutrino
masses may provide a clue to a still undiscovered realm of physical theory.
VI. NATURE: A WONDERFUL MYSTERY
Nature has written a wonderful mystery. The plot continually changes and the most
important clues come from seemingly unrelated investigations. These sudden and drastic
changes of scientific scene appear to be Nature’s way of revealing the unity of all fundamental
science.
The mystery begins in the middle of the nineteenth century with the puzzle: How does
the sun shine? Almost immediately, the plot switches to questions about how fast natural
selection occurs and at what rate geological formations are created. The best theoretical
physics of the nineteenth century gave the wrong answer to all these questions. The first hint
of the correct answer came, at the very end of the nineteenth century, from the discovery of
radioactivity with accidentally darkened photographic plates.
The right direction in which to search for the detailed solution was revealed by the 1905
discovery of the special theory of relativity, by the 1920 measurement of the nuclear masses
of hydrogen and helium, and by the 1928 quantum mechanical explanation of how charged
particles get close to each other. These crucial investigations were not directly related to
the study of stars.
By the middle of the twentieth century, nuclear physicists and astrophysicists could
calculate theoretically the rate of nuclear burning in the interiors of stars like the sun. But,
just when we thought we had Nature figured out, experiments showed that fewer solar
neutrinos were observed at earth than were predicted by the standard theory of how stars
shine and how sub-atomic particles behave.
At the beginning of the twenty-first century, we have learned that solar neutrinos tell us
not only about the interior of the sun, but also something about the nature of neutrinos.
No one knows what surprises will be revealed by the new solar neutrino experiments that
are currently underway or are planned. The richness and the humor with which Nature has
written her mystery, in an international language that can be read by curious people of all
nations, is beautiful, awesome, and humbling.
15
BIBLIOGRAPHY
[1] F. W. Aston, “The Mass-Spectra of Chemical Elements,” Philosophical Magazine and Journal
of Science 39, 611–625 (1920). In the course of a systematic program to measure the masses
of atoms, Aston found that four hydrogen nuclei (protons) are heavier than a helium nucleus
(an alpha particle) and two positive electrons [see Eq. (1)]. This fundamental discovery is the
experimental basis of our understanding of how stars like the sun shine. The original paper
is rarely cited, perhaps because the text is mainly devoted to a description of Aston’s new
apparatus and to a discussion of the many different masses that he measured. The hydrogenhelium mass difference is only briefly discussed.
[2] R. D. E. Atkinson and F. G. Houtermans, “Zur Frage der Aufbaum¨
oglichkeit der Elements in
Sternen,” Z. Physik 54, 656 (1929). An early attempt to calculate the rate of nuclear reactions
in stars using the Gamow factor.
[3] J. N. Bahcall, “Solar Neutrinos I. Theoretical,” Phys. Rev. Lett. 12, 300 (1964).
[4] H. A. Bethe, “Energy production in Stars,” Phys. Rev. 55, 436 (1939). If you are a physicist
and only have time to read one paper in the subject, this is the paper to read.
[5] J. D. Burchfield, Lord Kelvin and The Age of the Earth, (Chicago: University of Chicago Press,
1990). This concise book provides a clear and insightful account of Kelvin’s views on the age
of the earth and the age of the sun, and on many other topics including natural selection and
geological evolution. The author tells an exciting story with historical accuracy.
[6] C. L. Cowan Jr., F. Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire, “Detection of
the Free Neutrino: a Confirmation,” Science 124, 103 (1956); F. Reines and C. L. Cowan,
“Detection of the Free Neutrino,” Phys. Rev. 92, 830 (1953). These papers describe the first
experimental detection of neutrinos.
16
[7] C. Darwin, On the Origin of the Species by Natural Selection, or, The Preservation of Favored
Races in the Struggle for Life (London: Murray, 1859), p. 285 [Pelican Preprint of first edition,
296–297, 1968].
[8] R. Davis Jr., “Solar Neutrinos. II. Experimental,” Phys. Rev. Lett. 12, 302 (1964).
[9] J. N. Bahcall and R. Davis Jr., “An Account of the Development of the Solar Neutrino Problem,” in Essays in Nuclear Astrophysics, ed. C. A. Barnes, D. D. Clayton, and D. Schramm
(Cambridge: Cambridge University Press, 1982), p. 243; reprinted in J. N. Bahcall, Neutrino Astrophysics, (Cambridge: Cambridge University Press, 1989). For related material, see
http://www.sns.ias.edu/ jnb/Papers/Popular/snhistory.html.
[10] A. S. Eddington, “The Internal Constitution of the Stars,” Observatory 43, 353 (1920). This
lecture is inspiring.
[11] A. Einstein, “Zur Elektrodynamik bewegter K¨
orper,” Annalen der Physik 17, 891–921 (1905).
English translation in The Principle of Relativity, translated by W. Perrett and G. B. Jeffery
with notes by A. Sommerfeld, (New York: Dover Publications, 1923). The logic in this paper
is breathtakingly beautiful and incredibly clear.
[12] E. Fermi, “Tentativo di una teoria della emissione di raggi β,” Ric. 4, 491 (1934). Reprinted
in Enrico Fermi, Collected Papers: Note e memorie, Vol. 1, p. 538 (University of Chicago
Press: Chicago, 1962-1965). See also p. 559, 575. Fermi formulated the mathematical theory
of neutrino emission in β-decay. His first paper on the subject was rejected as “too speculative”
for publication.
[13] W. A. Fowler, “Experimental and theoretical nuclear astrophysics: the quest for the origin of
the elements,” Rev. Mod. Phys. 56, 149 (1984).
[14] G. Gamow, “Zur Quantentheorie der Atomzertr¨
ummerung,” Zeit. fur Physik 52, 510 (1928).
Derives the Gamow factor using quantum mechanics.
[15] S. Hawking, “Gravitationally collapsed objects of very low mass,” Monthly Notices of Royal
17
Astronomical Society 152, 75 (1971). In this imaginative paper, Hawking speculated that the
central region of the sun might contain a black hole and that this could be the reason why the
flux of solar neutrinos was less than predicted.
[16] H. von Helmholtz, Lecture “On the interaction of natural forces,” K¨
onigsberg, February 7,
1854, in Phil. Mag. 11 [series 4], 489–518 (1856).
[17] J. F. W. Herschel, A Treatise on Astronomy (London, 1833), p. 211.
[18] W. T. Kelvin, “On the Age of the Sun’s Heat,” Macmillan’s Magazine, 288–293 (March 5,
1862).
[19] J. Marchant, Alfred Russel Wallace, Letters and Reminiscences, I (London: Cassell, 1916), p.
242. Letter dated 14 April, 1869.
[20] W. Pauli, letter to a physicists’ gathering at T¨
ubingen, December 4, 1930. Reprinted in Wolfgang Pauli, Collected Scientific Papers, ed. R. Kronig and V. Weisskopf, Vol. 2, p. 1313 (Interscience: New York, 1964).
[21] H. N. Russell, “On the Sources of Stellar Energy,” Pub. Ast. Soc. Pacific, August (1919).
If you like to read mystery stories and to figure out “Who did it” from limited clues, then
you will love this paper. A year before Aston’s measurements of the mass of hydrogen and
of helium and two decades before Bethe’s calculations of nuclear fusion rates, Russell used
well-known observations of stars and simple physical reasoning to infer that the rate of the
“unknown process” that supplies stellar energy must increase rapidly with increasing stellar
temperature. Incredibly, he also correctly deduced that this dependence of energy production
on temperature would lead to stars being stable over very long periods of time. These insights
are presented in the text of a closely-reasoned lecture that contains no equations.
[22] E. Rutherford, “The Radiation and Emanation of Radium,” Pt. II, Technics, Aug., 171, (1904)
Collected Papers, I: 650.
[23] C. Smith and M. N. Wise, Energy and Empire: A biographical study of Lord Kelvin, (Cam-
18
bridge: Cambridge University Press: 1989). This book is a stimulating and authoritative
account of Kelvin, his science, and his life. Chapters 15-17 deal with the age of the sun, the
cooling of the earth, and the age of the earth.
¨
[24] C. F. von Weizs¨
acker, “Uber
Elementumwandlungen in Innern der Sterne. II,” Physikalische
Zeitschrift 39, 633 (1938). The CNO cycle is described in the last paragraph of Section 7.
19
APPENDIX A: THE CNO CYCLE
For stars heavier than the sun, theoretical models show that the
CNO (carbon-nitrogen-oxygen) cycle of nuclear fusion is the dominant source of energy generation. The cycle results in the fusion
of four hydrogen nuclei (1 H, protons) into a single helium nucleus
(4 He, alpha particle), which supplies energy to the star in accordance with Einstein’s equation. Ordinary carbon, 12 C, serves as a
catalyst in this set of reactions and is regenerated. Only relatively
low energy neutrinos (ν)are produced in this cycle. The figure
is adapted from J. N. Bahcall, Neutrinos from the Sun, Scientific
American, Volume 221, Number 1, July 1969, pp. 28-37.
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APPENDIX B: THE P − P CHAIN REACTION
21
In theoretical models of the sun, the p − p chain of nuclear
reactions illustrated here is the dominant source of energy
production. Each reaction is labeled by a number in the upper left hand corner of the box in which it is contained. In
reaction 1, two hydrogen nuclei (1 H, protons) are fused to
produced a heavy hydrogen nucleus (2 H, a deuteron). This
is the usual way nuclear burning gets started in the sun. On
rare occasions, the process is started by reaction 2. Deuterons
produced in reactions 1 and 2 fuse with protons to produce
a light element of helium (3 He). At this point, the p − p
chain breaks into three branches, whose relative frequencies
are indicated in the figure. The net result of this chain is the
fusion of four protons into a single ordinary helium nucleus
(4 He) with energy being released to the star in accordance
with Einstein’s equation. Particles called ‘neutrinos’ (ν) are
emitted in these fusion processes. Their energies are shown in
the figure in units of millions of electron volts (MeV). Reactions 2 and 4 were not discussed by Hans Bethe. This figure is
adapted from J. N. Bahcall, Neutrinos from the Sun, Scientific
American, Volume 221, Number 1, July 1969, pp. 28-37.
22